That was my experience with Foxit after years of using it with no issues.
One day a update arrived. I installed it.
A whole nest of adbars were installed on each browser I have on this system, FIrfox(primary)
IE hardly used and Chrome even less used.
In each case I found it impossible to totally remove the adbar. IT returned after reboots on Firefox,
and in the case of chrome I never did figure out how to remove it. IT always returned.
At the time I was using Norton Security, it was of course useless . That prompted me to install
Malware Bytes, it found the adware that it properly listed as a malware.
Removed it and I solved the problem. Note it did require a reboot and ANOTHER run of Malware
Bytes to totally eradicate.
I'll never use Foxit again. I've moved on to Sumatra. I have no idea if it is secure or not but
it works for the PDF's I read so far.
For those that say adbars addons like that are not malware, you are most likely part of the scum bag industry doing that sort of thing. It is malware if it acts like malware.